Apparatus providing redundancy for fabricating highly reliable memory modules

ABSTRACT

A method and apparatus for repair of a multi-chip module, such as a memory module, is provided, where at least one redundant or auxiliary chip attach location is provided on the substrate of the multi-chip module. The auxiliary chip attach location preferably provides contacts for attachment of more than one type of replacement semiconductor chip. Accordingly, when one or more chips on the multi-chip module are found to be completely or partially defective, at least one replacement chip can be selected and attached to the auxiliary location to provide additional memory to bring the module back to its design capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/832,053,filed Apr. 10, 2001, now U.S. Pat. No. 6,531,772 B2, issued on Mar. 11,2003, which is a continuation of application Ser. No. 09/443,080, filedNov. 18, 1999, now U.S. Pat. No. 6,215,181 B1, issued on Apr. 10, 2001,which is a continuation of application Ser. No. 08/728,302, filed Oct.8, 1996, now U.S. Pat. No. 6,008,538, issued on Dec. 28, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to single in-line memory modules(SIMMs), dual in-line memory modules (DIMMs), and the like, and, morespecifically, to SIMM, DIMM and other memory module boards providing atleast one extra die site for attachment of an additional die to replacea defective die or dice thereon.

2. State of the Art

An integrated circuit (IC) typically includes a semiconductor die (die)electrically attached to a lead frame providing physical support for thedie and connecting the die to external circuitry, such as a printedcircuit board or other conductor-carrying substrate. In such anarrangement, the lead frame and die may be connected by wire bonding thelead fingers of the lead frame to contact or bond pads located on asurface of the die. The die and lead frame are then typicallyencapsulated within a transfer-molded plastic package, although ceramicand metal packages may also be used, depending on the operatingenvironment and the packaging requirements of the die.

As the demand for memory, in particular random access memory (RAM),surpassed the memory capability of a single die, multi-chip modules(MCMs) were developed, such modules having a number of memory devicesattached to a single substrate, such as a printed circuit board. A SIMMis a memory module having multiples of the same basic die, where thesemiconductor memory chips are aligned in a row and interconnected to aprinted circuit board to, in effect, create a single device with thememory capacity of the combined memory chips. An example of a SIMM,including a plurality of dynamic random access memory devices (DRAMs)used as memory in a computer, is illustrated in U.S. Pat. No. 4,992,850,issued Feb. 12, 1991, to Corbett et al., assigned to the assignee of thepresent invention. As the demand for additional memory on a singledevice has increased, other devices, such as dual in-line memory modules(DIMMs), have also been developed. Such devices, while providing thedesired memory capability on a single printed circuit board, presentunique problems for the manufacturer when one or more of thesemiconductor memory chips thereon fail.

It is well known that semiconductor dice have an early failure rate,often referred to in reliability terms as “infant mortality.” Moreover,infant mortality of MCMs is multiplied depending on the number ofindividual semiconductor dice provided therein. For example, a SIMMcomposed of ten dice, each die having an individual reliability yield of95%, would result in a first pass test yield of less than 60%, while aSIMM composed of twenty dice, each die having an individual reliabilityyield of 95%, would produce a first pass test yield of less than 36%.

When a single packaged die, such as a dual in-line package (DIP), fails,a manufacturer can attempt to repair the device, use the device for somereduced capacity function if the device is only partially defective, orscrap it. When complete failure of a die has not occurred and a portionof the memory is good (e.g., 1, 2, or 3 megabits of a 4 megabit chip),such a device is not typically useful. For MCMs such as a SIMM, where anumber of semiconductor dice are attached to a single substrate,however, it may not be possible to use the device for some reducedcapacity function and it is surely not desirable to scrap the entire MCMwhen some, if not most, of the dice attached thereto are not defective.Thus, the manufacturer is left with the somewhat costly process ofreworking the MCM, typically by removing the defective chips andreplacing them with new ones. Such a procedure is described in U.S. Pat.Nos. 5,239,747 and 5,461,544, where a SIMM having a specialized tracepattern suitable for both burn-in and individual die testing is testedto determine if any of the semiconductor devices mounted thereon arenon-functional and, if so, either the defective device is replaced witha device which has been subjected to burn-in, or the entire multi-chipmodule can be subjected to another burn-in process after the replacementof the defective device. The defective devices, however, are merelyreplaced by removing the defective device and replacing it with another,either a device previously subjected to burn-in or not. This reworkprocess can be complicated, time consuming and costly, depending uponthe type of device, the type of mounting of the device on the substrate,and the type of substrate used for mounting. For example,plastic-packaged devices are typically physically pulled to disconnecttheir leads from the module, while so-called “glob topped” (silicone orepoxy gel covering) dice may be removed after cutting through theencapsulant to the wire-bonded die, which is pulled. In addition, sincereplacing multiple unacceptable dice on an MCM poses physical risks toother MCM dice during the replacement operation, it may be desirable todiscard such an MCM rather than attempt rework, particularly where thereliability of the replacement die is not known.

Depending on the extent of testing and/or burn-in procedures employed, adie may typically be classified into varying levels of reliability andquality. For example, a die may meet only minimal quality standards byundergoing standard probe testing or ground testing while still in waferform, while individual separated or “singulated” dice may be subjectedto intelligent burn-in at full-range temperatures with full testing ofthe die's circuitry. A die that has been so tested is termed a “knowngood die” (KGD). Examples of methods for the testing and burn-in of anindividual die prior to packaging are disclosed in U.S. Pat. Nos.5,448,165 and 5,475,317.

A cost-effective method for producing known reliable SIMMs, DIMMs andthe like with larger numbers of chips on a single device is desirablefor industry acceptance and use. In an attempt to provide known reliableSIMMs complying with consumer requirements, it would be desirable tofabricate the SIMM completely of KGD. Using only KGD in a SIMM, however,would not currently be cost effective since each KGD has to be subjectedto performance and burn-in testing, both of which are costly at thispoint in time. Typically, however, SIMMs are fabricated fromprobe-tested dice, and are subsequently burned-in and performancetested. In contrast to the use of all KGD in a SIMM, when using dicewith well known production and reliability histories, particularly wherethe dice being used are known to have a low infant mortality rate, theuse of such minimally tested dice to produce a SIMM is usually found tobe the most cost effective alternative.

As previously stated, since typical testing and burn-in procedures aregenerally labor and time intensive, posing significant risks to the diceof a SIMM, in the event that a SIMM contains an unacceptable die,replacement of the unacceptable die with a KGD is preferable. Modulerework with a KGD does not typically require the SIMM to be subjected toadditional burn-in procedures that can unnecessarily stress the dice. Anexample of a method and apparatus for the testing and burn-in of anindividual die prior to packaging is illustrated in U.S. Pat. No.5,424,652, issued Jun. 13, 1995, to Hembree et al., assigned to theassignee of the present invention. Such a method and apparatus provide asource of KGD to allow for the rework of an unacceptable die in an MCMwith a KGD. In other instances, it is known to test a die in a packagefor functionality and replace any defective die. Such is illustrated inU.S. Pat. Nos. 5,137,836, 5,378,981, and 5,468,655.

One way in the art to eliminate the need to physically remove defectiveor unacceptable dice from a SIMM has been to provide additional,redundant spaces on the printed circuit board for attachment ofreplacement chips. Thus, one additional space has been provided adjacenteach memory chip on the board, the additional spaces providing contactsfor attachment of a semiconductor chip similar to the one it isreplacing. For example, if a 32 megabit SIMM contains eight 4 megabitchips, then eight additional spaces are provided on the SIMM, configuredto accept up to eight additional 4 megabit chips, if necessary. Such aconfiguration, however, results in a memory module that is approximatelytwice as big as a memory module having no extra spaces.

Therefore, a need exists for the cost-efficient fabrication of SIMMs,DIMMs, and the like, of known performance and reliability requirementsthat requires a minimal amount of rework when one or more dice attachedthereto are found defective.

BRIEF SUMMARY OF THE INVENTION

Accordingly, a memory module, such as a single in-line memory module(SIMM) or dual in-line memory module (DIMM), is provided having at leastone redundant or auxiliary chip attach location for attachment of areplacement chip. When one or more dice on a memory module are founddefective, one or more replacement chips can be attached to the one ormore auxiliary chip attach locations with the size of the replacementchips being at least equal to the amount of defective memory. Thus, thedefective dice can be replaced without needing to be physically removed.Moreover, by providing auxiliary chip attach locations that can acceptdifferent sizes and memory capacities of replacement semiconductorchips, one replacement chip can replace several defective chips on thememory module.

In a preferred embodiment, a SIMM or DIMM board is provided having aplurality of primary chip attach locations and one auxiliary chip attachlocation. Each of the plurality of primary chip attach locations issimilarly configured to accept the same type of semiconductor chip, suchas a number of 4 megabit chips. The auxiliary chip attach location, onthe other hand, is configured to accept more than one capacity ofreplacement semiconductor chip. Thus, depending on the amount ofdefective memory detected on the SIMM, a replacement chip having atleast that amount of good memory can be attached to the auxiliary chipattach location. Consequently, the replacement chip may be a 1 megabitchip, if only one 1 megabit of memory is found defective, or a 4 megabitchip, if an entire 4 megabit chip is found to be defective, theconfiguration of the auxiliary chip attach location being capable ofaccepting either replacement chip.

In another preferred embodiment, a SIMM or DIMM is provided having tworows of the same type of semiconductor memory chip and a redundant oradditional chip attach location for accepting a variety of semiconductormemory chips. The redundant chip attach location is electronicallyconnected in parallel to the rest of the memory chips so that if one ormore memory chips are found defective, an auxiliary replacement chiphaving an amount of memory approximately equal to that found defectivecan be attached to the additional chip attach location.

Having the capability to easily and cost effectively rework memorymodules without the need to remove defective chips or the need tosubstitute defective chips on a one to one basis is highly desirable.The ability to provide auxiliary chip attach locations that canaccommodate a plurality of different chip configurations not only makesrework more simple, but allows memory modules with large numbers ofchips to be cost effective. For example, in yet another preferredembodiment, a memory module having three rows of similar memory chips isprovided with more than one auxiliary chip attach location, each of theauxiliary chip attach locations being capable of receiving more than onetype of semiconductor die. Thus, if only one of the many chips providedthereon fails entirely, then one substantially identically configuredchip can be attached to any one of the three auxiliary chip attachlocations. If more than one memory chip fails or more defective memoryis located than can be replaced with a single auxiliary chip then, ifnecessary, more than one replacement chip, can be attached to any one ormore of the three auxiliary chip attach locations.

In yet another preferred embodiment, rather than having all of theauxiliary chip attach locations capable of receiving variouslyconfigured chips, at least two auxiliary chip attach locations are eachprovided with different configurations. Thus, depending on the number ofbad memory chips, one or more chips having a combined memory capacitysubstantially equal to the bad memory can be attached to the auxiliarychip attach locations. For example, on a DIMM with five defective 4megabit chips found, a replacement 16 megabit chip can be attached tothe auxiliary chip attach location configured to receive 16 megabitchips and a replacement 4 megabit chip can be attached to the auxiliarychip attach location configured to receive a 4 megabit chip.

Preferably, the replacement chips are KGD so that the additional burn-inis not required on the memory module. Moreover, the KGD may be partiallydefective dice or “partials” that are known to be good for a certaincapacity of memory (e.g., 3 megabits of a 4 megabit chip). This isparticularly attractive, since a high percentage (approaching 50%) forsome designs of 16 megabit DRAMS is partially or completely defective,while 5-10% of 4 megabit DRAMS comprise partials. Since partials mightotherwise be discarded, beneficial use thereof as replacement chipsenhances the effective yield rate for the chips and lowersper-unit-memory costs.

Thus, for example, for SIMMs having a design memory capacity of 32megabits with 7 of the 32 megabits tested defective, a replacementpartial 16 megabit chip known to be good for 7 megabits could beattached to an auxiliary location. The defective memory of 7 megabits onthe primary chip(s) could then be disabled. Similarly, for a 32 megabitmemory module with 7 bad megabits of memory and more than one auxiliarychip attach location, a replacement full 4 megabit KGD and a 3 megabit“partial” KGD could be attached to two of the auxiliary chip attachlocations.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of a memory modulehaving an auxiliary chip attach location according to the presentinvention;

FIG. 1A is a schematic side view of a chip that has been flip-chipbonded to a substrate;

FIG. 1B is a schematic side view of a chip that has been wire bonded toa substrate;

FIG. 2 is a close-up schematic view of a second embodiment of anauxiliary chip attach location;

FIG. 3 is a schematic view of the memory module of FIG. 1 including areplacement chip attached to the auxiliary chip attach location;

FIG. 4 is a close-up schematic view of a third embodiment of anauxiliary chip attach location;

FIG. 5 is schematic view of a second embodiment of a memory modulehaving an auxiliary chip attach location according to the presentinvention;

FIG. 6 is a schematic view of a third embodiment of a memory modulehaving three auxiliary chip attach locations, two of which havereplacement chips attached thereto according to the present invention;

FIG. 7 is a schematic view of a fourth embodiment of a memory modulehaving two auxiliary chip attach locations according to the presentinvention;

FIG. 8 is a schematic view of a memory card in accordance with thepresent invention; and

FIG. 9 is a diagrammatical view of an electronic system utilizing amemory module in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a single in-line memory module (SIMM) 10, inaccordance with the present invention, is illustrated having an elongatesubstrate 12, such as a printed circuit board (PCB) or other substrateknown in the art to which a plurality of semiconductor memory chips 14is attached. The chips 14 are all preferably substantially similar inconfiguration and memory capacity and are flip-chip bonded to individualchip attach locations of the substrate 12 as is known in the art. Thatis, as shown in the example of flip-chip bonding of FIG. 1A, the chips14 are provided with bumped solder balls 15 on each bond pad 17 locatedon the active surface 19 of the chip 14 and are superimposed oversimilarly configured contacts or terminals 21 on the surface 16 of thesubstrate 12, at which time the solder balls 15 are heated and melted or“reflowed” to form a mechanical and electrical connection between thesubstrate 12 and each chip 14. Other conductive elements, such as aconductive or conductor-filled epoxy, may be employed in lieu of solder.Each chip 14 may also be wire bonded to the substrate 12, as illustratedin FIG. 1B, where, as opposed to FIG. 1A, the chip 14 is positioned onactive surface 19 and wire bonds 23 are made between the bond pads 17 ofthe chip 14 and contacts or terminals 21 on the surface 16 of thesubstrate 12.

Longitudinally extending along one edge 18 of the SIMM 10, a malesocket-type electrical connection 20 is provided having a plurality ofelectrical contacts 22 configured to interface with a SIMM socket asknown in the art. Each of the chips 14 are electrically connected to oneor more of the plurality of electrical contacts 22 through variouselectrical traces carried in or on the substrate 12, as known in theart.

In addition to the standard or primary chip attach locations for thechips 14, a redundant or auxiliary chip attach location 24 is providedon the surface 16. The auxiliary chip attach location 24 is providedwith a plurality of contact points or terminals 26 that can be connectedto more than one type of replacement semiconductor chip. As shown inFIG. 1, the auxiliary chip attach location 24 is provided with an innerarray 28 of contact points or terminals 26 and an outer array 30 ofcontact points or terminals 26. As better illustrated in the close-upview of FIG. 2, the auxiliary chip attach location 40 includes aplurality of contacts 42 comprising an inner array 44 for attachment ofa chip of similar configuration to the chips 14 of FIG. 1. An outerarray 46 of contacts 42 forms the appropriate configuration forattachment of a larger-capacity replacement chip. In the illustratedembodiment, each of the contacts 42 of the inner array 44 is connectedvia a trace 48 to a corresponding contact 42 of the outer array 46.While in some instances such a one-to-one correspondence between thecontacts 42 of the inner and outer arrays 44 and 46 may be desirable,such a configuration is not required. The configuration of the outerarray 46, for example, may depend on the type, size, and configurationof a replacement die to be connected to the outer array 46.

Accordingly, if during intelligent burn-in of the SIMM 10 one of the tenchips 14 shown in FIG. 1 completely fails, then a replacement chip ofsubstantially similar configuration to chips 14 can be attached to theinner array 28 to replace the failed chip 14. As illustrated in FIG. 3,however, if more than one chip 14 fails or memory equaling the capacityof more than one chip is proven defective during burn-in, it may benecessary to connect a larger-capacity replacement chip 50 to theauxiliary chip attach location, one that provides enough memory toreplace the combined memory of the failed chips 14.

It should be noted that the replacement chip may be attached to themodule by a technique different from that used to connect chips 14.Thus, the replacement chip may be wire-bonded for ease of attachment,while the chips 14 were flip-chip attached by solder reflow.Thereplacement chip may then be separately glob-topped or otherwiseprotected after wire bonding, the primary chips having been previouslyunderfilled and encapsulated during initial fabrication of the module.

Referring now to FIG. 4, another preferred embodiment of a chip attachlocation 60 is illustrated. The single array 62 of the chip attachlocation 60 is comprised of a plurality of elongate contacts 64outwardly extending from an inner perimeter 66 to an outer perimeter 68defined by the inner and outer ends 70 and 72 of each contact 64,respectively. Such a configuration allows attachment of variously sizedreplacement chips 50, so long as the bond pads of the replacement chip50 can be bonded to the contacts of the chip attach location 60, whetherby wire bonding such as that illustrated in FIG. 1B, by flip-chipbonding as illustrated in FIG. 1A, or by other methods known in the art.

As illustrated in FIG. 5, a SIMM or dual in-line memory module (DIMM) 80including two rows of memory chips 84 can also benefit from having aredundant chip attach location 82 for attachment of an additional orreplacement chip if any of the chips 84 provided thereon are founddefective. In prior art devices, a SIMM or DIMM 80 having twenty memorychips 84 thereon, as illustrated in FIG. 5, would typically not prove tobe cost effective without the aforementioned 100% redundancy ofauxiliary to primary chip sites. Having a redundant chip attach location82 that can accommodate various sizes of replacement memory chipsprovides an easy, cost effective means of reworking such a device.

Similarly, in FIG. 6, a memory module 90 having thirty individual memorychips 92 is made possible by providing at least one auxiliary chipattach location 94. Because of the difficulty associated with reworkinga device having so many potentially defective chips, manufacturing sucha device as memory module 90 would typically not even be attemptedwithout the exclusive use of pretested known-good-die (KGD). Theexclusive use of KGD primary chips would clearly not be cost effectivebecause of the cost associated with testing each chip individuallybefore attaching it to the memory device.

As illustrated in FIG. 6, it may be desirable to provide more than oneauxiliary chip attach location 94 such as auxiliary locations 96 and 98.Such a configuration has added benefits because it provides moreflexibility for the type and number of chips that can be attachedthereto. For example, if four of the memory chips 92 are foundcompletely defective, each of the chips 92 having a design memorycapacity of 16 megabits, and another chip 92 exhibits 3 megabits ofdefective memory, then a 3 megabit (partial 4 megabit) chip 100 can beattached to auxiliary location 98 and a 64 megabyte chip 102 attached toauxiliary location 96. Various other combinations could also be deviseddepending on the KGD on hand and the memory size of those KGD.

As shown in FIG. 7, it is contemplated that more than one auxiliary chipattach location 112 and 114 be provided on the memory device 110, eachof the auxiliary locations 112 and 114 capable of accepting a differentreplacement chip. In this preferred embodiment, the single auxiliarylocation 82 as shown in FIG. 5 has been divided into two auxiliarylocations 112 and 114. Accordingly, depending on the number, if any, ofdefective or bad chips 116, various combinations of known-goodreplacement chips can be selected depending on the size and memory sizeof those on hand.

It is also contemplated that various other memory modules known in theart, such as the DRAM card 120 illustrated in FIG. 8, could include atleast one auxiliary chip site 122 for replacement of defective memory ofprimary chips 124. Moreover, those skilled in the art will appreciatethat, while not specifically illustrated, other multi-chip modules mayalso include and benefit from at least one redundant or auxiliary chipattach location.

Finally, as shown in FIG. 9, the multi-chip modules of the variouspreferred embodiments herein described, such as SIMM 10 of FIG. 1, canbe incorporated into a memory device 130 of an electronic system 132,such as a computer system, that includes an input device 134 and anoutput device 136 coupled to a processor device 138. Of course, themulti-chip module 10 can alternatively be incorporated into the inputdevice 134, the output device 136, or the processor device 138.

It will be appreciated by those skilled in the art that various methodscan be used to achieve the desired memory capability of the memorydevice, such as enabling and/or disabling devices 118 located on eachchip 116 of FIG. 7. The enabling and/or disabling devices 118 mayinclude fuses, antifuses, and other such devices known in the art thatcan fully or partially disable the memory capacity of a chip 116.Furthermore, a programmable device, such as a so-called “trafficcontrol” EEPROM, as known in the art, may be installed on each moduleand programmed based on burn-in results to reroute input and outputpaths and link the new KGD auxiliary chips to the remainder of memory onthe module so as to present an interface to the mother board or otherhigher-level packaging, which is indistinguishable to the host systemfrom a perfect as-fabricated module with 100% good primary or originalmemory. Such a traffic control EEPROM is illustrated at 86 in FIG. 5.

Those skilled in the art will also appreciate that the number andconfiguration of auxiliary chip attach locations may vary, depending onthe configuration of replacement chips and the needs of the user.Further, while the invention has been described with relation to memorydevices, the invention may be practiced on many other multi-chip moduleswhere a single chip attach location having the capability to acceptvarious semiconductor devices could reduce the time and cost associatedwith rework.

It should be noted that the term “chip,” as used in the specificationand appended claims, is intended as exemplary and not limiting, theinvention having applicability to any packaged die, bare die, and/or anyintermediate product thereof. In addition, while the preferredembodiments were illustrated as being flip-chip bonded to the substrate,the dice, whether replacement or otherwise, may be wire bonded orotherwise electrically attached to the substrate as known in the art.

It will also be appreciated by one of ordinary skill in the art that oneor more features of any of the illustrated embodiments may be combinedwith one or more features from another to form yet another combinationwithin the scope of the invention as described and claimed herein. Thus,while certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the invention disclosedherein may be made without departing from the scope of the invention,which is defined in the appended claims.

What is claimed is:
 1. A memory module, comprising: a conductor-carryingsubstrate; at least two sites on the substrate respectively mounting atleast two memory chips providing a design memory capacity for the memorymodule; and one other site on the substrate for mounting another memorychip thereon of a functional capacity ranging from less than that of oneof the at least two memory chips to greater than that of one of the atleast two memory chips but no greater than that of the at least twomemory chips.
 2. A multi-chip memory module, comprising: aconductor-carrying substrate having a plurality of memory chips mountedthereon; and a programmable device adapted to reroute input and outputpaths to the memory chips to avoid defective memory present in at leastone of the plurality of memory chips while utilizing functional memorypresent in the at least one of the plurality of memory chips and toincorporate functional memory placed at one or more additional locationson the substrate into the rerouted input and output paths to replace thedefective memory.
 3. The module of claim 2, wherein at least one of theplurality of memory chips exhibits defective memory, and furthercomprising at least one additional memory chip mounted at one of the oneor more additional locations and providing functional memory at leastequivalent in amount to the defective memory.
 4. The module of claim 3,wherein the at least one additional memory chip itself includes anamount of defective memory.
 5. The module of claim 3, wherein the atleast one additional memory chip comprises at least two additionalmemory chips placed at different additional locations, at least two ofthe at least two additional memory chips being of different memorycapacity.
 6. The module of claim 2, wherein the programmable devicecomprises an EEPROM.
 7. A multi-chip memory module comprising: aplurality of memory chips mounted to a conductor-carrying substrate, theplurality of memory chips collectively exhibiting an amount of defectivememory in excess of a memory capacity of any individual memory chipamong the plurality; and at least one additional memory chip providingan amount of functional memory capacity at least equal to the amount ofdefective memory.
 8. The module of claim 7, wherein the at least oneadditional memory chip comprises at least two memory chips havingdifferent amounts of functional memory.